The present invention relates to an optical scanning system.
The optical scanning system is well known to be assembled with a laser printer and used recently as an optical system of various electronic apparatuses such as a digital copying apparatus and a facsimile apparatus. Also, the optical scanning system is intended to be applied to a display unit or a measuring device.
The optical scanning system has a light deflection means which generally comprises a rotary polygon mirror or a galvanomirror which has a deflection mirror surface to deflect a light. In the optical scanning system in cooperated with such a deflection means, it is necessary to arrange an F-theta (f .theta.) lens between the deflection means and a scanning surface to be optically scanned in order to maintain the scanning speed constant on the scanning surface and prevent uneven pitch of the scanning lines due to a tilt error of the deflection mirror surface.
On the other hand, the optical scanning system is always required to be small and simple in construction and economical in producing cost.
In order to make the optical scanning system small, it is necessary to widen the angle of field of the lens system arranged between the deflection means and the scanning surface so as to scan a long range with a short optical path length. In order to simplify the whole structure of the optical scanning system and reduce the cost thereof, it is necessary to simplify the structure of the lens system arranged between the deflection means and the scanning surface.
Various attempts for simplifying the lens system arranged between the deflection means and the scanning surface have been proposed so far for the purpose of realizing a small and simple optical scanning system at a low cost.
For example, Japanese Unexamined Patent Publication (Kokai) No. 54-87540 discloses an optical sca nning system in which a divergent bundle of rays emitted from a light source is introduced into a lens system arranged between the deflection means and the scanning surface. However, this optical system is imapired by a curvature of field (image surface curvature) which is more than 5 mm for a half field angle of 15 to 17 degrees with respect to a focal length of 150 mm. Therefore, this known optical system is not satisfactory from the stand point of widening the field angle of the lens system.
Also, Japanese Unexamined Patent Publication (Kokai) No. 61-48468 discloses a F-theta lens composed of a single lens element. However, this lens is also impaired by a curvature of field wnhich is 5 to 10 mm for a half field angle of 21 degrees with respect to a focal length of 300 mm. Therefore, this proposed F-theta lens is not satisfactory from the stand point of widening the field angle of the lens as in the case of the publication previously mentioned.